The present patent application claims foreign priority benefits under 35 U.S.C. xc2xa7119 to Italian patent application No. SV2001A000016, filed May 28, 2001, now pending.
The invention relates generally to a variable gain amplifier for received signals in ultrasound or nuclear magnetic resonance imaging apparata comprising at least one amplifier unit having at least one input and at least one output and at least one feedback circuit which connects one of the output signals with a feedback input.
Conventional variable gain amplifiers of this general type are known in the art, in which a feedback circuit, which connects an output with a feedback input, uses resistive elements to define the amplification factor.
FIG. 1 shows prior art, with the amplification factor being given by the ratio between the values of the two resistors R1/R2. In this case, high amplification factors are easily obtained, by properly selecting a high value for R1. Nevertheless, this generates a high thermal noise which is a troublesome drawback in the particular application of ultrasound or nuclear magnetic resonance signal amplification, due to the low power of the useful received signal.
On the other hand, this specific application wherefor the amplifier is designed, defines the ideal characteristics thereof. Hence, for ultrasound and nuclear magnetic resonance imaging applications, the amplifier must have a wide band and high amplification factors, while keeping noise levels to a minimum. It shall be noted that the received signals to be amplified have a strength of a few microvolt and frequencies of the order of radio-frequencies. These signals should be typically amplified by a few tenths of db. All the above listed specifications tend to be in contrast with each other, i.e. any intervention aimed at meeting one of the specifications of the amplifier impairs the other required conditions. Moreover, since these received signals have predetermined frequencies, the amplifier should be further allowed to operate in a linear manner over received signals having frequencies within a predetermined range.
An additional requirement of the specific field of application for the present invention consists in obtaining a variable gain for the amplifier. This allows to compensate for any natural attenuation of the train of signals received along a scan line of an ultrasound probe. The illuminating pulses transmitted by the probe are reflected at different depths of the body under examination, and the reflected signals are naturally attenuated to an extent related to the depth of the body under examination whereat they are generated as a reflection of the pulses emitted by the probe. Therefore, attenuation is a function of reflection depth and, as a result, a function of time. This attenuation, when not properly compensated for by a corresponding variation of the amplification ratio, would introduce errors in the evaluation of received signals.
Particularly, the feedback circuit of an amplifier causes a negative feedback or reverse feedback, wherein the output signal is drawn by a special feedback circuit and properly processed and is applied to the feedback input where said processed output signal is subtracted, via a suitable mixer circuit, from the input signal applied to the amplifier.
An advantage provided by the use of negative feedback in an amplifier is that the amplifier gain may be stabilized with respect to the variations of the characteristic parameters of active elements (transistors, FET, MOSFE, operational amplifiers, electronic tubes) used in the amplifier. Hence, if the amplifier gain without the negative feedback circuit is sufficiently high, the application of said circuit proves to cause the amplifier gain to only depend from the parameters of the negative feedback circuit. Another important advantage deriving from the use of negative feedback is the considerable increase of frequency response and linearity in the amplifier which uses negative feedback as compared with an amplifier having the same gain as that of the amplifier which uses negative feedback, but obtaining said gain without using a negative feedback circuit.
If the amplifier is intended to have a constant gain with respect to the width of the signal applied to the input, then the negative feedback circuit will be formed by linear elements, such as resistors, capacitors and inductors. If this is the case the amplifier gain will only depend on the values of these elements.
However, if the amplifier is intended to have a variable gain with respect to the width of the signal applied to the input, then the negative feedback circuit must have at least one non linear element.
The simplest method to apply a nonlinearity in the negative feedback circuit consists in using an element which has a variable resistance, the latter being a function of the voltage applied to this element and/or to the current which flows through it. For instance, by using a properly connected diode in the negative feedback circuit, a variable gain amplifier may be obtained.
The use of nonlinear elements, such as diodes, in the negative feedback circuit, introduces a resistive nonlinear relationship between the input signal and the output signal of the negative feedback circuit, thereby causing a strongly nonlinear operation of the whole amplifier. Moreover, the presence of diodes over the path of the negative feedback signal or over the path of the main signal causes an increase of the amplifier noise, and hence a decrease of the signal-to-noise ratio of the amplifier.
The present invention is based on the problem of providing a variable gain and low noise amplifier for received signals in ultrasound or nuclear magnetic resonance imaging apparata like the one described hereinbefore, in such a manner that, by simple, inexpensive and safely operating arrangements, the above drawbacks of well-known devices may be obviated, and the needs of the specific field of application may be satisfactorily accounted for, while suppressing or minimizing the drawbacks deriving from the fulfillment of contrasting requirements or specifications.
The present invention solves the above problems by providing a variable gain and low noise amplifier for received signals in ultrasound or nuclear magnetic resonance imaging apparata like the one described herein, in which at least one feedback circuit comprises at least one capacitive divider.
The capacitive divider is made in such a manner that the selected capacitance values provide constant reactance values at the operating frequencies. This provides low resistive values and high amplification factors. Therefore, the drawback caused by the high thermal noise generated by purely resistive negative feedback circuits is obviated, while reaching the required high amplification factors. Further, the particular selection of capacitance values ensures that the amplifier response is linear and within the band of frequencies designed for the received signals to be amplified.
By using a capacitive divider, the physiological time-related attenuation of the received signals, to be amplified may be also automatically accounted for. The invention achieves this result by changing the amplification ratio in a manner that is related and opposite to attenuation, in order to compensate for it. A particular embodiment of the amplifier allows to perform this automatic compensation of the amplification factor without affecting the signal-to-noise ratio. This is possible thanks to the fact that the resistive behavior is constant and unaffected by the amplification factor, and with the operating point of the amplifier always being in the linear range.
With reference to a negative feedback circuit having a capacitive divider, the amplification factor A is given by
|A|=|Z1(f)|/|Z2(f)|
where
|Z1,2(f)|=|R1,2+1/xc1,2(f)|
in which xc1,2 (f) are the reactance values depending on the frequency (f) of the received signals to be amplified.
A variable gain amplifier for received signals in imaging apparata according to one embodiment of the present invention comprises at least one amplifier unit having at least one input and at least one output and at least one feedback circuit which connects one of the output signals with a feedback input, characterized in that at least one feedback circuit comprises at least one capacitive divider.
One object of the present invention is to provide an improved variable gain amplifier.
Related objects and advantages of the present invention will be apparent from the following description.